The primary source of money is the Jobs plugin. The Jobs plugin permits you to select a Job that conforms to your playstyle and make money from typical Minecraft gameplay. A player may only work one Job at a time, but can swap between them at will. Switching from a Job will induce a level penalty for that Job, which reduces your income. You can regain that experience by working that Job again.
/jobs browse - Opens a GUI that displays information about each Job and allows you to pick one.
Miners, as one might expect, make their money by mining. Spending time deep underground or at contested ore nodes will prove lucrative for you.
Builders make money by placing blocks, simple. You could make money building a beautiful city, a fleet of airships, or designing advanced redstone systems. You may find meaningful work as a real estate developer for a Town or as a shipwright.
Explorers make money passively by finding new chunks and performing archeology. Explorer makes less money per hour than most other Jobs, but can make money entirely passively by entering chunks they have not entered before.
Anglers make money by fishing and performing other aquatic work. A relaxing profession that might draw you to a coastal Town or Nation where your contribution to the local food supply will be heavily desired.
A catch-all Job for any agriculturally-mined Minecrafter. Make money farming, breeding animals, raising bees, or logging. It may not be much, but is honest work.
A Job that makes money exclusively by combat. Makes more money per hour than most other Jobs, but comes with risk. Fighters can earn money by killing other players, and even by engaging in Movecraft combat. Fighters often find use for their services in large Towns or Nations with political ambition.